High-resolution observation of live cells on silicon nitride film using a conventional optical microscope

Optical microscopes are essential equipment for observing and analysing cultured cells. In observations using a normal optical microscope, as the contrast of cells is rather low, phase contrast and differential interference microscopes are commonly used. In some cases, specific proteins in cells are fluorescently labelled and observed using a fluorescence microscope. In such observations, cells are generally cultured on a glass surface and observed through the glass. Therefore, spherical aberration occurs due to the refraction of light on the glass surface. This spherical aberration becomes larger when using an objective lens with higher magnification and higher numerical aperture, resulting in a decrease of spatial resolution and contrast. Here, we found that by observing cells cultured on an extremely thin silicon nitride (SiN) film (50 nm thick) under a conventional optical microscope, we could directly observe the cells at high resolution without spherical aberration. This improvement of spatial resolution was confirmed with both inverted and upright optical microscopes. Using a normal 100 × objective lens without oil immersion, we were able to directly observe melanosomes in melanoma cells and analyse their dynamic movement. We believe that using our original sample holder with thin SiN film will make it easy to observe live cells at high resolution and that this method will contribute to a wide range of biological research.